Display module

ABSTRACT

A display module includes a pixel disposed in respective intersections between a plurality of scanning lines and a plurality of data lines, which lines are aligned in a matrix, and a current supply line that supplies electric current to the pixel, the pixel includes a data storage device for storing a data signal supplied from the data lines, and an emitting device that emits light by the electric current supplied by the current supply line according to the data signal stored in the data storage device. One terminal of the data signal storage device is connected to one of the scanning lines placed anteriorly of to the pixel that is being selected.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. Ser. No.10/105,273, filed Mar. 26, 2002, the contents of which are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to an active matrix display module,especially, pertaining to such display module as being provided withpixels comprising such emitting devices as an EL (electro luminescence)device or a LED (light emitting diode) wherein light is emitted byconducting electric current through such an emitting layer as an organicsemiconductor thin film and with pixels circuits to control the emittingoperation of those pixels.

BACKGROUND OF THE INVENTION

Recently, the advent of advanced information network system in ourmodern life increases the demand for personal computers, car navigationsystems, portable remote terminals and communication network equipmentor the combined products thereof. For such products as mentioned above,such a display device as being thin in thickness, light in weight and oflower power consumption is suitable so that a liquid crystal display ora display module incorporating such electrooptic devices as aself-luminous EL device or LED therein are in practical use.

The latter display module is characterized in being good at visibility,possessed with wide viewing angle and being suitable for motion displaydue to its high response, which module is said to especially opt forimage display. In particular, a display incorporating an organic ELdevice (also referred to as an organic LED device; hereinafter, referredto as OLED in abbreviation, where appropriate), the emitting layer ofwhich device is made from organic matter, is highly expectable inaccordance with the greater improvement of the luminous efficiencythereof and the progress of the network technology that enhances thefeasibility of image communication. The OLED display is of diodestructure in which an organic emitting layer is interposed between twopieces of electrodes.

As described below, the active matrix driving method in which thin filmtransistors (hereinafter, referred to as TFT where appropriate) serve asthe switching devices of pixels is effective for enhancing the powerefficiency of such OLED display as mentioned above. The operation of theOLED display by the active matrix deriving method in the prior arts isdisclosed in the Japanese Patent Application Laid-open Nos. HEI 4-328791and 8-241048 or the U.S. Pat. No. 5,550,066, for instances, the drivingvoltage of which display is disclosed in the International PublicationNo. WO98-36407 and so forth.

The typical pixel structure of the OLED display comprises two TFTsequivalent to a first active device and a second active device, thefirst of which is a switching transistor while the second of which is adriver transistor, and a pixel driving circuit (hereinafter, referred toas a pixel circuit where appropriate) comprising one storage capacitancefor a data signal storage device, which pixel circuit controls theemitting luminance of the OLED display. The pixels are disposed at therespective intersections of a matrix in which the data lines having M innumber to which data signals (or image signals) are supplied and thescanning lines (hereinafter, referred to as gate lines whereappropriate) having N in number are aligned in the arrangement of Nlines x M rows.

To drive the pixels, the scanning signals (gate signals) aresubsequently supplied to the gate lines having N in number so as to putthe switching transistors into on-state condition, and one time of thevertical scanning operation is over within one frame period of Tf so asto freshly supply the on-state voltage to the first gate line.

In the above driving method, the time required for supplying theon-state voltage to one gate line is defined as Tf/N or below.Generally, the value of the one frame period of Tf is in the order of1/60 second in practical use. During the on-state voltage is supplied toa certain gate line, all the switching transistors that are connected tothe corresponding data line are put into on-state condition, insynchronization with which the data voltage (image voltage) issimultaneously or subsequently supplied to the data lines having M rowsin number, which arrangement is general in the active matrix liquidcrystal display.

The data voltage, during the on-state voltage is supplied to the gateline, is stored in storage capacitance (capacitor) so as to bemaintained substantially as it is during the one frame period. Thevoltage value of the storage capacitance regulates the gate voltage ofthe driver transistor.

Accordingly, the value of the current flowing through the drivertransistor is controlled so as to put the light emission of the OLEDunder control. The response time for the OLED to begin emitting lightupon the application of voltage thereto normally takes one μs or belowso that it manages to follow the motion image of quick tempo.

The active matrix driving method realizes high efficient operation byperforming light emission over the one frame period. The difference withthe passive matrix driving method, in which the respective electrodes ofthe OLED diode are directly connected to the scanning line and the dataline without the provision of any TFTs, is clear as follows.

In the passive matrix driving method, the current flows through the OLEDonly during the short period when the scanning line is selected, so thatemitting luminance corresponding to practically several times as many asthe number of the scanning lines is required for obtaining the sameluminance as that of the one frame period of the active matrix drivingmethod with the light emission performed during such short period asmentioned above. Thus, it is unavoidable that the driving voltage andcurrent in use enlarge, which causes large loss of power consumptionsuch as heating so as to deteriorate power efficiency.

In view of the foregoing, it is found that the active matrix drivingmethod is superior to the passive one in light of the reduction of powerconsumption.

SUMMARY OF THE INVENTION

The active matrix driving OLED display module is arranged such thatcurrent supply to capacitors to keep display over one frame period iscarried out by connecting one electrode thereof to an output terminal ofthe switching transistors while connecting the other electrode thereofto either a common voltage line exclusively used for the capacitors or acurrent supply line that supplies current to an OLED.

FIG. 8 is a block diagram to show one example of the prior OLED displaymodule while FIG. 9 is an explanatory view to show the pixel arrangementof the module as shown in FIG. 8. This display module 1 (image displaymodule) is arranged on a substrate 6 made from such insulating materialas glass such that a data driving circuit 3, a gate driving circuit 4and a current supply circuit 5 are disposed in the surrounding of adisplay section 2 as surrounded by the dotted line in the drawing, inwhich section the plurality of data lines 7 and the plurality of gatelines 8 are aligned in a matrix.

The data driving circuit 3 includes a complementary circuit of anNP-channels TFT or a shift register circuit, a level shifter circuit andan analog switch circuit comprising a single (N or P) channel TFT. Tonote, the current supply circuit 5 may be modified into just a bus linewhile the current may be supplied from an external current supplysource.

FIG. 8 shows a system in which a common voltage line 9 designed for thecapacitors is provided in the display section 2, to which common voltageline 9 the other electrodes of the capacitors are connected. The commonvoltage line 9 is led out to an external common voltage source from aterminal 11 of a common voltage supply bus line 9A.

As shown in FIG. 9, a pixel 20 comprises a TFT 21 or a switchingtransistor, a TFT 22 or a driver transistor, a capacitor 23 and an OLED24, which are disposed in the region surrounded by the data line 7 andthe gate line 8. The gate of the TFT 21 is connected to the gate line 8while the drain thereof is connected to the data line 7. The gate of theTFT 22 is connected to a source of the TFT 21, to which source oneelectrode (positive pole) of the capacitor 23 is connected.

The drain of the TFT 22 is connected to a current supply line 10 whilethe source thereof is connected to the organic light emitting diode OLED24. The other electrode (negative pole) of the capacitor 23 is connectedto the common voltage line 9. The data line 7 is driven by the datadriving circuit 3 while the scanning line (or gate line) 8 is driven bythe gate driving circuit 4. A current supply line 10 is connectedthrough the current supply bus line 10A to the current supply circuit asshown in FIG. 1.

In FIG. 9, when the TFT 21 is put into on-state condition upon theselection of one pixel 20 by the gate line 8, a video signal suppliedfrom the data line 7 is stored in the capacitor 23. Then, when the TFT21 is put into off-state condition, the TFT 22 turns into on-statecondition so that the current flows from the current supply line 10 tothe OLED 24, which current keeps intact practically over one frameperiod. The current in conduction at this time is regulated by a signalcharge stored in the capacitor 23.

The operation level of the capacitor 23 is regulated by the electricpotential of the common voltage line 9, which controls the lightemitting behavior of the pixel.

The above system requires the provision of the common voltage line 9that gets through a portion of the pixel region, which causes thedeterioration of a so-called aperture ratio and constraints thedisplaying brightness of the module on the whole from improving.

FIG. 10 is a block diagram to show another example of the prior OLEDdisplay module in the similar arrangement to that as shown in FIG. 9.The basic alignment of the TFT 21 and TFT 22 and the capacitor 23comprising the respective pixels is the same as shown in FIG. 9, thedifference with which lies in the fact that the other end of thecapacitor 23 is connected to the current supply line 10.

That is, when the TFT 21 is put into on-state condition upon theselection of one pixel 20 by the gate line 8, a video signal suppliedfrom the data line 7 is stored in the capacitor 23, and when the TFT 22turns into on-state condition while the TFT 21 is put into off-statecondition, the current flows from the current supply line 10 to the OLED24, which current keeps intact practically over one frame period in thesame way as the example shown in FIG. 9. The current in conduction atthis time is regulated by a signal charge stored in the capacitor 23,and the operation level of the capacitor 23 is regulated by the electricpotential of the current supply line 10, which controls the lightemitting behavior of the pixel.

In the above prior example, there is likelihood that not only dot defectbut also vertical and horizontal line defect might occur, when shortcircuit is caused in the upper and lower electrodes of the capacitor 23so as to cause large electric current owing to the fact that the currentsupply line connected to the capacitor 23 is situated at a high-voltageside of the substrate.

The present invention is to provide a display module incorporating suchan electrooptic device as the OLED therein, the arrangement of whichmodule allows the aperture ratio of the pixels to improve and to preventsuch display fault as line defect so as to display image with highquality as well as avoids the increase of the number of productionprocess thereof.

In order to achieve this purpose, an active matrix display module isprovided herein wherein the plurality of scanning lines and theplurality of the data lines ate aligned in a matrix, which modulecomprises a pixel in respective intersections between said scanninglines and data lines and a current supply line that supplies electriccurrent to the pixels for display, wherein said pixel comprises anactive device that is selected by the scanning line, a data storagedevice that stores a data signal that is supplied from the data linewhen the active device is put into on-state condition and an emittingdevice that emits light by the electric current supplied from thecurrent supply line according to the data signal as stored in the datastorage device, wherein a reference voltage of the data storage devicefor driving the emitting device over a fixed period is a voltage of thescanning line placed immediately anterior to the pixel that is beingselected.

This arrangement allows the aperture ratio of the pixels to improve andimage to be displayed with high quality as well as avoids the increaseof the number of the production steps thereof so as to reduce theproduction cost. The more concrete arrangements of the display moduleaccording to the present invention are described below.

-   -   (1) The display module of the present invention comprises a        pixel disposed in the respective intersections between the        plurality of scanning lines and the plurality of data lines,        which lines are aligned in a matrix, and a current supply line        that supplies the electric current to the pixels, wherein said        pixel comprises a pixel circuit provided with a first active        device and a second active device as well as a data signal        storage device, a pixel electrode in which an emission and        non-emission of the pixel is controlled by a signal output by        the pixel circuit and a common electrode, wherein an output        terminal of the first active device is connected to an input        terminal of the second active device and one terminal of the        data signal storage device is connected to a point where the        output terminal of the first active device and the input        terminal of the second active device connect, wherein the first        active device is switched into one of on-state and off-state        condition by a gate signal supplied from the scanning line        wherein in the on-state condition thereof a data signal supplied        from the data line is stored in the data signal storage device,        wherein the second active device in the off-state condition of        the first active device supplies electric current from the        current supply line to the pixel electrode according to the data        signal stored in the data signal storage device, wherein the        other terminal of the data signal storage device is connected to        a scanning line, to which the gate signal of the pixel electrode        scanned anterior to the scanning line to switch the first active        device is supplied.    -   (2) In the arrangement (1), the current supply lines are        provided in parallel with the data lines.    -   (3) In the arrangement of either (1) or (2), the first and        second active devices are thin film transistors and the data        signal storage device is a capacitor.    -   (4) In any one of the arrangements (1) to (3), the gate driving        circuit that supplies gate signals to the plurality of scanning        lines, the data driving circuit that supplies data signals to        the plurality of data lines and the current supply circuit that        supplies electric current to the current supply lines are        provided in the substrate in which the scanning lines, the data        lines and the current supply lines are provided.    -   (5) In any one of the arrangements (1) to (4), an emitting layer        interposes between the pixel electrode and the common electrode        comprising the pixel.    -   (6) In the arrangement (5), the emitting layer is an organic        emitting layer.

It should be appreciated that the present invention is not limited tothe above arrangements and the embodiments as described below, but itcan be modified into various manners within the scope of theaccompanying patent claims. The other objects and arrangements of thepresent invention are clarified from the following description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram to show the arrangement of the firstembodiment of the display module according to the present invention.

FIG. 2 is a circuit view to show the detailed arrangement of one pixelof the module as shown in FIG. 1.

FIG. 3 is a plan view in the vicinity of one pixel to show one exampleof the display module according to the present invention.

FIG. 4 is a plan view in the vicinity of one pixel to show anotherexample of the display module according to the present invention.

FIG. 5 is a view to show the formation step of the respective structurallayers of the display module as shown in FIG. 3.

FIG. 6 is a view to show the formation step of the respective structurallayers of the display module as shown in FIG. 3 following the step asshown in FIG. 5.

FIG. 7 is a view to show the formation step of the respective structurallayers of the display module as shown in FIG. 3 following the step asshown in FIG. 6.

FIG. 8 is a block diagram to show the arrangement of one example of theprior OLED display module.

FIG. 9 is a view to show the pixels arrangement of the display module asshown in FIG. 8.

FIG. 10 is a block diagram like FIG. 9 to show the arrangement ofanother example of the prior OLED display module.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, the preferred embodiments of the present invention aredescribed in details with reference to the accompanying drawings. Theorganic emitting layer of the respective pixels as described below isdivided into such types as emitting light with luminance in proportionto the value of the current and with a color (including white) inherentin an organic material in use and incorporating a red, green, blue orother color filter into the organic layer to emit white light so as todisplay color image.

FIG. 1 is a block diagram to show the arrangement of the firstembodiment of the display module according to the present inventionwhile FIG. 2 is a circuit view to show the detailed arrangement of onepixel of the module as shown in FIG. 1. In the present embodiment, inthe same way as shown in FIG. 10, data lines 7, scanning lines 8 andcurrent supply lines 10 are provided in the display region 2. In thedrawings, the scanning lines 8 are represented with 8 (n+1), 8 (n)placed anterior thereto and 8 (n−1) placed farther anterior theretowhile the data lines 7 are represented with 7 (m+1), 7(m) and 7(m−1).

For the sake of conveniences, the scanning line as being scanned (orselected) is regarded as the 8 (n+1) in the following description.Attention is paid to the Pixel 20 among the plurality of pixels asselected by the scanning line 8 (n+1). The first TFT 21 and the secondTFT 22 that are active devices are a switching transistor and a drivertransistor respectively. The gate of the first TFT 21 is connected tothe scanning line 8 (n+1) and the drain thereof is connected to the dataline 7 (m+1) while the source thereof is connected to the gate of thesecond TFT 22.

The drain of the second TFT 22 is connected through a current supplyline 10A to a current supply line 10. The source thereof is connected toan anode of an OLED 24. One terminal of a capacitor 23 for a data signalstorage device is connected to a point where the source of the first TFT21 and the gate of the second TFT 22 connect while the other terminalthereof is connected to the scanning line 8 (n) placed anterior to the 8(n+1).

In the circuit arrangement of one pixel as shown in FIG. 2, one terminalof the capacitor 23 that is connected to the point where the source ofthe first TFT 21 and the gate of the second TFT 22 connect is ofpositive pole while the other terminal thereof as connected to thescanning line 8 (n) is of negative pole. The OLED 24 is arranges suchthat an organic emitting layer, which is not shown in the drawings,interposes between the anode 25 and the cathode 26 thereof, the anode 25being connected to the source of the second TFT 22 and the cathode 26being connected to an electrode provided on the side of the same andcomprising the pixel, which electrode is not shown in the drawings.

In the above arrangement, the driving operation of the respective pixelsfor display, which pixels are in connection with the prior scanning line8 (n), is over wherein the TFTs (the first TFTs) thereof are onoff-state condition over most of the one frame (or one field) period.That is, the electric potential of the scanning line 8(n) is on a givenlow voltage state, which state does not fluctuate during most of the oneframe (or field) period.

Thus, the capacitor 23 of the pixel in connection with the scanning line8 (n+1) as being scanned stably stores a data signal as written thereinfrom the data line 7 (m+1), which allows the electric current thatconducts into the OLED 24 through the second TFT 22 from the currentsupply line 10 to correctly correspond to the data signal as stored inthe capacitor. This also applies to all the pixels that are inconnection with the scanning line 8 (n+1).

The arrangement hereof allows the voltage of the capacitor 23 for thedata signal storage device to keep intact, so that there is no casewhere the reference voltage of the capacitor fluctuates owing to theinflow of the electric current into the OLED that occurs in the priordisplay module as shown in FIG. 10, with the result that there is nofluctuation of the light emitting quantity of the OLED, which allowsimage to be displayed with high quality. Further, the riddance of thecommon voltage line of the display module as shown in FIG. 8 or 9improves the aperture ratio of the pixel and avoids the increase of thenumber of the production steps thereof, which brings the production costthereof to reduction.

Then, the arrangement of the display module according to the presentinvention is more concretely described below.

The data signal (image signal etc.) may be supplied to the data line inanalog or time sharing digital quantity, and the system in which thearea of the respective pixels is divided may be incorporated. Normally,the pixel circuit is produced by the low temperature polysilicone TFTtechnique. In this case, it is preferred that the electrodes comprisingthe capacitor utilize a layer of the channel and gate portions of thetransistor. The channel portion thereof is spaced apart from the gateportion thereof by a thin insulating layer, which allows an area forsecuring the capacitance as required to be rendered narrower.

FIGS. 3 and 4 show a plane view of the vicinity of one pixel of thedisplay module according to the present invention. In the drawings,reference numerals 7, 8 (n) and 8 (n+1), and 10 indicate a data line,scanning lines, and a current supply line respectively. One pixel issurrounded by the scanning lines 8(n) and 8 (n+1) and the current supplyline 10, in which the first TFT 21, the second TFT 22, the capacitor 23,the anode 25 of the OLED as indicated with the dotted line, a contacthole 27 for connecting the data line 7, the scanning lines 8(n) and8(n+1) as well as the current supply line 10 to the respective TFTs andthe capacitor 23, an aperture 28 to receive an organic emitting layertherein are included.

The pixel circuit is surrounded by the current supply line 10 to theOLEDs, which line is disposed vertically with regard to the drawings inthe same direction as the data line 7 to supply image signal and assuch, and the gate lines 8(n) and 8(n+1) as disposed horizontally withregard thereto. The switching transistor for the first TFT 21 writes thedata signal supplied from the data line into the capacitor 23 of thepixel circuit.

The dual gate system is adopted for the first TFT 21 in order to abatethe decrease of the storage charge of the capacitor 23 owing to leakingcurrent. The driver transistor for the second TFT 22 is disposed infolds in order to obtain a comparatively large gate length. Thecapacitor 23 is disposed under the wiring system in order to reduce theaffect of the pixel upon the aperture 28. The other pole (the otherterminal) of the capacitor 23 is electrically connected to the gate linethat is scanned anterior to that being scanned.

There are two methods for the connection thereof to the prior gate lineaccording to which electrode (terminal) of the capacitor 23 is connectedthereto. FIG. 3 shows the case where the lower electrode thereof isconnected thereto while FIG. 4 shows the case where the upper electrodethereof is connected thereto.

Generally, it is required to make either of the electrode thereofconductive for the purpose of forming a capacitor by overlaying twoelectrodes one over another. The lower electrode thereof is made fromthe same material as that for the channel portion of the transistor, sothat it is basically required to perform doping operation on the wholesurface thereof by such method as ion implantation for the purpose ofmaking the whole surface thereof conductive. Such conductive material asmetal is normally adopted for the upper electrode thereof.

The ion implantation onto the whole surface of the lower electrodethereof should be performed before the formation of the upper electrodethereof, which is not desirable because it increases the number ofproduction process. The polarity of the electrodes thereof is taken intoconsiderations in the present embodiment in order to dispense with theion implantation fro rendering the whole surface of the lower electrodethereof conductive. It is known that the movement of carrier occurswithout performing the ion implantation on the whole surface thereof soas to serve as a capacitor, presuming that where the n-type ion dopingis performed is a low-tension side while where p-type ion doping isperformed is a high-tension side. Accordingly, the ion implantation isnot performed on the whole surface thereof, which is performed after theformation of the upper electrode thereof in this embodiment.

Provided that the first TFT 21 for the switching transistor is of n-typetransistor, the voltage applied to the gate line is on low voltage oroff-state condition during most of the frame period and turns intoon-state condition only during the writing operation of the data signal.In this case, it is preferable that where the lower electrode thereof isconnected to the prior gate line, the n-type ion implantation isperformed while the p-type ion implantation is performed where the upperelectrode thereof is connected thereto.

In the arrangement as shown in FIG. 3, the other terminal of thecapacitor 23 of the pixel under scanning is connected through a contacthole 27 as shown therein to the prior gate line 8(n).

In the arrangement as shown in FIG. 4, the gate line 8(n) extends overto the capacitor portion of the 8 (n+1) so as to be used as an upperelectrode of the same. Thus, the switching transistor is connected tothe lower electrode thereof.

FIGS. 5 through 7 show the formation steps of the respective structurallayers of the display module as shown in FIG. 3, the references (A) to(I) in which drawings indicate the formation of the respectivestructural layers. The layers as formed by the respective steps areshown with visible outlines.

At (A) in FIG. 5, a polysilicone layer 30 for the channel portion of thetransistor and the lower electrode of the capacitor is formed on a glasssubstrate not shown in the drawing, on which layer laser annealing orheating operation is performed for crystallization. Then, a firstinsulating layer is formed of p-TEOS and as such on the annealed orheated layer.

At (B) in FIG. 5, a conductive layer 80 for the scanning lines, the gateof the transistor and the upper electrode of the capacitor is formed oftitanium Ti or tungsten W and as such. The n-type ion implantation isperformed thereon by using the upper electrode thereof as a mask layout.

At (C) in FIG. 5, a second insulating layer is formed on the conductivelayer 80, on the required portions of which layer contact holes 270 aremade by means of the wet etching method and so forth.

At (D) in FIG. 6, the drain of the transistor and the current supplyline are formed with an aluminum wiring 70, the upper and lower sides ofwhich wiring may be sandwiched with titanium Ti or tungsten W and assuch.

At (E) in FIG. 6, a third insulating layer is formed on the aluminumwiring so as to form a contact hole 271 for an anode.

At (F) in FIG. 6, ITO is coated thereon for an anode of the OLED, onwhich patterning is performed so as to form an anode 25.

At (G) in FIG. 7, a passivation film is coated on the anode 25, throughwhich file a hole is made so as to provide an aperture 28 for receivingan organic emitting layer, which aperture corresponds to the emittingregion of the module.

At (H) in FIG. 7, an organic emitting layer 260 is coated on theaperture 28, which layer is coated on the region of the apertureincluding the circumferential edge thereof. For color display, anorganic emitting layer to emit a given color light is coated on theaperture of the respective pixels.

At (I) in FIG. 7, a cathode 26 to cover the organic emitting layer 260is formed so as to obtain a basic panel substrate. Thereafter, thispanel substrate is sealed by an appropriate sealing structure so as tobe put into a module form. Through the above production steps, thedisplay module embodied in the present invention is obtained.

It should be noted that the present invention is not limited to the OLEDdisplay module, but is also applicable to another display module thatemits light in the similar manner to the OLED module.

As described above, according to the present invention, the occurrenceof line defects resulting from the short-circuit between the electrodesof the capacitor is prevented and the increase of the number ofproduction process is avoided, so that the display module, the apertureratio of the respective pixels of which module is improved so as toallow image to be displayed with high luminance and quality, isprovided.

1. Display module comprising: a pixel disposed in respectiveintersections between a plurality of scanning lines and a plurality ofdata lines, which lines are aligned in a matrix; and a current supplyline that supplies electric current to the pixel; wherein the pixelincludes a data storage device for storing a data signal supplied fromthe data lines, and an emitting device that emits light by the electriccurrent supplied by the current supply line according to the data signalstored in the data storage device; and wherein one terminal of the datasignal storage device is connected to one of the scanning lines placedanteriorly of to the pixel that is being selected.
 2. Display moduleaccording to claim 1, wherein the data storage device is a capacitor. 3.Display module according to claim 2, wherein the one terminal of thecapacitor is connected through a polysilicon layer to the one of thescanning lines placed anteriorly of to the pixel that is being selectedor the data lines.